Capacitor discharge ignition systems apply the charging voltage to a capacitor through a diode. The charge stored on the capacitor is discharged through the primary of the ignition transformer when a silicon controlled rectifier (SCR) is fired at the appropriate time. Ignition has been terminated by shorting the capacitor to ground. The switch is exposed to the high voltage on the capacitor. Any leakage or shorting at the switch due to high ambient moisture (and/or dirt) conditions will result in reduced voltage across the spark plug or in shorting out the ignition system.
The time the shutdown switch is exposed to high voltage can be reduced by connecting the switch between the charging coil side of the diode and ground so the switch is exposed to only pulses of high voltage instead of the longer duration charge on the capacitor. When so connected, the switch would, however, be subjected to a high negative voltage spike not used in the CD ignition system. To overcome that problem, U.S. Pat. No. 3,964,461 provides two diodes in series between the charging coil and the capacitor and connects the shutdown switch between the diodes so the diodes act to protect the switch against high negative spikes and against the charge on the capacitor. That arrangement charges the capacitor through two diodes which inherently increases the charge time and increases (doubles) the power loss. This reduces the voltage across the spark plug.